There are many uses for systems that cause fluid to flow through channels, including microfluidic channels, i.e., channels having at least one dimension in the micron range (less than one millimeter). For instance, there are many examples of systems for analyzing very small amounts of samples and reagents on chemical “chips” that include very small fluid channels and small reaction/analysis chambers. Small-scale systems are currently being developed for genetic analysis, clinical diagnostics, drug screening, and environmental monitoring. These systems must handle liquid or gas samples at very small scales, and must be compatible with chip-based substrates. Microfluidics, the behavior of fluid flow in very small-scale systems, therefore is central to the development of these systems. Many of these systems also require use of electrical circuits. In conventional devices, the microfluidic components and the electrically conductive pathways and components are separate structures, which can be difficult to combine and integrate together into a single device.
There are several established techniques for making metal microstructures in three dimensions. Electroplating and electroless deposition are standard methods for constructing microstructures with metallic layers several nanometers to several microns thick in two- or three-dimensions (Schlesinger, M. and M. Paunovic, eds., Modern Electroplating, New York: John Wiley, 2000). This approach to has been used to join hand-assembled, two-dimensional components electrochemically (Jackman, R. J. B., S. T.; Whitesides, G. M., Fabrication of Three-Dimensional Microstructures by Electrochemically Welding Structures Formed by Microcontact Printing on Planar and Curved Substrates. Journal of Microelectromechanical Systems 1998, 7, (2), 261-266). This method has been used for rapid prototyping of optical masks (Wang, W. H., Holl, M. R., Schwartz, D. T. Rapid prototyping of masks for through-mask electrodeposition of thick metallic components. J. Electrochem. Soc. 2001, 148(5):C363-C368). Microcontact printing has also been combined with electroplating (Jackman, R. J.; Brittain, S. T.; Adams, A.; Prentiss, M. G.; Whitesides, G. M., Design and fabrication of topologically complex, three-dimensional microstructures. Science 1998, 280, (5372), 2089-2091 and Jackman, R. J.; Brittain, S. T.; Adams, A.; Wu, H. K.; Prentiss, M. G.; Whitesides, S.; Whitesides, G. M., Three-dimensional metallic microstructures fabricated by soft lithography and microelectrodeposition. Langmuir 1999, 15, (3), 826-836, or electroless deposition (Wu, H. K.; Whitesides, S.; Whitesides, G. M., Fabrication of micro-chain mail by simultaneous, patterned electrodeposition on a plane and multiple cylinders. Angewandte Chemie-International Edition 2001, 40, (11), 2059-2060 and Wu, H.; Brittain, S.; Anderson, J.; Grzybowski, B.; Whitesides, S.; Whitesides, G. M., Fabrication of topologically complex three-dimensional microstructures: Metallic microknots. Journal of the American Chemical Society 2000, 122, (51), 12691-12699), to pattern metal onto the surface of capillaries. This technique was used to fabricate freestanding, three-dimensional cages of metal. Patterned metal layers have also been released from a two-dimensional template to generate a foldable metal structures and free-standing objects (Brittain, S. T.; Schueller, O. J. A.; Wu, H. K.; Whitesides, S.; Whitesides, G. M., Microorigami: Fabrication of small, three-dimensional, metallic structures. Journal of Physical Chemistry B 2001, 105, (2), 347-350. Metal has also been deposited onto flat, non-conducting surfaces by treatment with electrolytes in microfluidic channels (Yan, J. D., Y.; Liu, J.; Cao, W.; Sun, X.; Zhou, W.; Yang, X.; Wang, E., Fabrication of Integrated Microelecrodes for Electrochemical Detection on Electrophoresis Microchip by Electroless Deposition and Micromolding in Capillary Technique. Analytical Chemistry 2003, 75, 5406-5412). A related technique has been used to form metal patterns on curved surfaces (LaVan, D. A. G., P. M.; Langer, R., Simple, Three-Dimensional Microfabrication of Electrodeposited Structures. Angewandte Chemie-International Edition 2003, 42, (11), 1262-1265). All of these methods appear to have been used exclusively to pattern smooth surfaces.
To generate solid replicas of three-dimensional objects, several investigators have used a technique referred to as ‘microcasting’ (Piotter, V.; Benzler, T.; Gietzelt, T.; Ruprecht, R.; Hausselt, J., Micro powder injection molding. Advanced Engineering Materials 2000, 2, (10), 639-642 and Chung, S.; Park, S.; Lee, I.; Jeong, H.; Cho, D., Replication techniques for a metal microcomponent having real 3D shape by microcasting process. Microsystem Technologies-Micro-and Nanosystems-Information Storage and Processing Systems 2005, 11, (6), 424-428. Techniques based on LIGA (Lithographie, Galvanoformung and Abformung) can produce metallic objects by deposition of a metal onto a three-dimensional, molded polymer template that is subsequently removed to yield an open structure (such as a honeycomb arrangement of open cells) (Arias, F.; Oliver, S. R. J.; Xu, B.; Holmlin, R. E.; Whitesides, G. M., Fabrication of metallic heat exchangers using sacrificial polymer mandrills. Journal of Microelectromechanical Systems 2001, 10, (1), 107-112 and Harris, C.; Kelly, K.; Wang, T.; McCandless, A.; Motakef, S., Fabrication, modeling, and testing of micro-cross-flow heat exchangers. Journal of Microelectromechanical Systems 2002, 11, (6), 726-735). However, LIGA and other conventional injection molding techniques require expensive equipment (including metal molds) and metals, such as gold, high pressure (3-5 MPa for low pressure powder injection molding; higher for other techniques), can result in undesirable shrinking of the molded metal upon cooling (typically 15-22%).
A growing interest in flexible displays has fueled the development of polymer-metal composites and other materials. These most conventional approaches and composites require a layer-by-layer approach to making laminated materials, and methods based on nanoparticles require annealing at temperatures up to 200° C.
In addition, magnetic components have been used in lab-on-a-chip systems. Magnets have formed the basis of microfluidic pumps, mixers, and valves, and have been integrated into microfluidic systems to trap and move paramagnetic particles (Deng, T.; Whitesides, G, M.; Radhakrishnan, M.; Zabow, G.; Prentiss, M. Manipulation of magnetic microbeads in suspension using micromagnetic systems fabricated with soft lithography. App. Phys. Lett. 2001, 78, 1775-1777 and Lee, C. S.; Lee, H.; Westervelt, R. M. Microelectromagnets for the control of magnetic nanoparticles. App. Phys. Lett. 2001, 79, 3308-3310), and to guide the self-assembly of particles into structures (Hayes, M. A.; Polson, N. A.; Garcia, A. A. Active Control of Dynamic Supraparticle Structures in Microchannels. Langmuir 2001, 17, 2866-2871). There are several biologically-related applications where magnetically fields may be useful, including, for example immunoassays, acceleration of the hybridization of DNA and RNA, digestion of proteins, and sorting biomolecules. In cell biology, magnets have been used to isolate cells from whole blood, extract genomic DNA from cells, and to move magnetotactic bacteria. The use of magnetics in microfluidic systems has been reviewed recently (Pamine, N. Magnetism and microfluidics. Lab Chip 2006, 6, 24-38).
Electromagnets can have certain advantages over permanent magnets because they can be switched on/off rapidly using electrical signals, and the strength of their field can be adjusted. Electromagnets have been included in microfluidic systems for the manipulation of superparamagnetic beads. For example, electromagnets have been fabricated surrounding a microfluidic chamber by electroplating copper wires around a nickel-iron core, and have been used to capture superparamagnetic beads in channels (Ahn, C. H.; Allen, M. G.; Trimmer, W.; Jun, Y.; Erramilli, S. A fully integrated micromachined magnetic particle separator. J. Microelectromech. Syst. 1996, 5, 151-158). Other investigators have utilized other methods for combining electromagnets and microfluidics (Deng, T.; Whitesides, G, M.; Radhakrishnan, M.; Zabow, G.; Prentiss, M. Manipulation of magnetic microbeads in suspension using micromagnetic systems fabricated with soft lithography. App. Phys. Lett. 2001, 78, 1775-1777; Lee, C. S.; Lee, H.; Westervelt, R. M. Microelectromagnets for the control of magnetic nanoparticles. App. Phys. Lett. 2001, 79, 3308-3310; Wirix-Speetjens, R.; Fyen, W.; Xu, K.; De Boeck, J.; Borghs, G. A force study of on-chip magnetic particle transport based on trapped conductors. IEEE Trans. Mag. 2005, 41, 4128-4133; Smistrup, K.; Hansen, O.; Bruus, H.; Hansen, M. F. Magnetic separation in microfluidic systems using microfabricated electromagnets-experiments and simulations. J. Mag. Mag. Mat. 2005, 293, 597-604); Choi, J.; Ahn, C. H.; Bhansali, S.; Henderson, H. T. A new magnetic bead-based, filterless bio-separator with planar electromagnet surfaces for integrated bio-detection systems. Sens. & Act. B 2000, B68, 34-39 and Lee, H.; Purdon, A. M.; Westervelt, R. M. Manipulation of biological cells using a microelectromagnet matrix. App. Phys. Lett. 2004, 85, 1063-1065).
While these examples describe a range of useful devices and techniques, a need exists for other types of microfluidic devices comprising conductive pathways, circuits, electromagnets, etc.